The better beamer is more like a snoot, and directs and magnifies the light in one direction. A soft box would diffuse the light in many directions.

The 580ex is going to give you more reach than the 430ex. Are you that far away that the 580ex wouldn't be effective? The better beamer will give you more reach, but in most situations that come to mind for an indoor auditorium I would not use it. The big lens draws enough attention on its on without adding the beamer to it. But to each his own on this one.